Neutral wire for power distribution systems

ABSTRACT

At neutral or messenger wire for a power distribution system comprises a plastics tube supported by metal wires extending along the outside of the tube. Optical fibers arc located within the plastics tube and a coating is applied to the outside of the tube to enhance wear characteristics.

[0001] The present invention relates to neutral wires for use in anabove ground power distribution systems.

[0002] Electrical power is distributed through conductors carried aboveground on poles. Typically, the system will have one or more lineconductors carrying AC at different phases and a neutral wire to providea return path.

[0003] The service requirements for a neutral wire require it to beelectrically conductive as well as self-supporting. In more recent yearsuse has been made of the neutral wire to also carry a messenger wire sothat information can be transmitted along the existing powerdistribution corridors. The wires have been located within a conductive,usually aluminum, tube which in turn is supported by wire strands thatprovide the tensile strength for the ground wire. It is also known toincorporate optical fibers with the messenger wire to increase theinformation carrying capacity.

[0004] Because the neutral wire is unsupported between the posts, it issubject to the normal mechanical forces due to its own inherent weight,wind and other atmospheric conditions such as ice build up. Thesemechanical forces are imposed on the wire strands and the aluminum tubewhich in turn transmit the forces through to the optic fibers.

[0005] To avoid the possibility of damage to any optical fibers used asmessenger wires, it is known to arrange the fibers helically on a spacerwithin the tube. Such an arrangement is relatively expensive due to theneed for the additional components and the need to assemble the fiberonto the spacer. As an alternative to a helical spacer it is also knownto ensure that an excess length of fiber is provided within the tube sothat as the tube is subjected to the elongation due to the mechanicalforces, the strain is not passed through to the fiber.

[0006] The use of an aluminum tube protects the messenger wire fromwater and other environmental concerns. However, the aluminum tube isrelatively weak in compression and therefore is unable to withstand theradial forces that might be imposed upon it during installation. Toprovide additional mechanical strength it has been proposed to utilizeconcentric rings of aluminum wires around the aluminum tube. Typicallythese wires are steel with an aluminum cladding adjacent the tube and analuminum alloy as a second band of wires. The increased tensile andradial strengths avoid damage to the tube but at the same time increasesthe expense of the wire.

[0007] It is therefore an object of the present invention to provide aneutral wire in which the above disadvantages are obviated or mitigated.

[0008] In accordance with one aspect of the present invention there isprovided a neutral wire for use in an above ground power distributionsystem. The neutral or messenger wire comprises an elongate plasticstube and at least one optical fiber freely moveable within said tube andhaving an overall length greater than the tube in a free body state. Aplurality of metal wire strands are disposed about and extend along thelength of said tube.

[0009] The provision of a plastic tube to house the optic fibers enablesthe requisite radial strength to be attained.

[0010] Preferably said tube is coated with a protective plastics coatingof a dissimilar material.

[0011] An embodiment of the invention will now be described by way ofexample only with reference to the accompanying drawings in which:

[0012]FIG. 1 is a schematic illustration of a power distribution system.

[0013]FIG. 2 is a perspective view of a portion of the ground wire usedin the power distribution system of FIG. 1.

[0014]FIG. 3 is an end view of the ground wire shown in FIG. 2.

[0015]FIG. 4 is an enlarged view of a portion of an alternativeembodiment of distribution system.

[0016] Referring therefore to FIG. 1, an electrical power distributionsystem generally indicated at 10 includes a pair of posts 12 supportingpower conductors 14. Each of the conductors 14 carries an AC supply andare at different phases. A neutral wire 16; also referred to as amessenger wire, extends between the posts 12 below the conductors 14 andis fixed at each of the posts 12 by suitable clamps.

[0017] The neutral wire 16 is shown in more detail in FIGS. 2 and 3 andincludes a plastics tube 18. A number of optic fibers 20 are locatedwithin the tube 18. The optic fibers are utilized to carry informationand typically the tube will carry a significant number of fibers,sometimes upwards of 200. The fibers 20 are loosely located within thetube 18 and, in a free body state, have a length greater than that ofthe tube. The excess length of the fibers 20 will be at least 0.05%greater than that of the tube 18 although greater discrepancies may beused. In this way the tube 18 may elongate without inducing acorresponding strain upon the fibers 20.

[0018] An outer film or coating 22 is applied to the tube 18 to provideenhanced protection for the outer surface of the tube.

[0019] A plurality of wire strands 24 are helically wound around theoutside of the tube 22. The strands 24 are typically a steel wire 26having an aluminum coating 28 or are galvanized. Alternatively thestrands are made entirely of an aluminum alloy. The strands 24 providethe requisite tensile strength for the tube to span between the posts 12and provide a conductive path for electrical current.

[0020] The tube 18 is formed from a plastics material having arelatively high modulus of elongation to permit extension greater than1% and typically in the order of 6%. The extension can be approximately10 times the excess length of the fibers 20. The tube is formed from anengineering thermoplastic, typically polycarbonate, polybutyleneterephtalate (PBT) polyethylene tetraphtalate (PET) or polyethylenenaphthalate (PEN). In the preferred embodiment, tube 18 has a diameterof between 3 and 6 millimeters and a wall thickness of between 0.5 and2.0 millimeter.

[0021] In a typical application the tube 18 has a diameter of 5.0 mm anda wall thickness of 0.8 mm.

[0022] The outer coating 22 is preferably a plastics material exhibitinggood wear characteristics and resistance to environmental factors.Preferably polyethylene is used for the coating and applied at athickness of 0.1 mm to 1.0 mm to give a diameter of between 3 and 7 mm.

[0023] The wire strands 24 are chosen to be of sufficient diameter andnumber to provide the tensile strength required for the span between theposts 12 and may if preferred be arranged in 2 annular bands.

[0024] In use, the plastic tube 18 has sufficient radial strength towithstand the radial loads imposed on the cable due to the clamping atthe posts. At the same time the plastic tube provides protection for thefibers from water and ice and the excess length of the fibers in thetubes prevents strain being induced in the fibers when the ground wireis subject to mechanical loading.

[0025] It will be appreciated that the dimensions noted above withrespect to the preferred embodiment are exemplary only and theindividual components may be adjusted to suit the particular mechanicalrequirements of a given installation.

[0026] A further embodiment is shown in FIG. 4 where like referencenumerals will be used to denote like components with a suffix ‘a’ addedfor clarity.

[0027] In the embodiment of FIG. 4, the conductors 16 a are suspendedfrom a neutral or messenger wire 14 a by a hanger 30. The hanger 30 hasspaced apertures 32 to receive one of the conductors 16 a and a hookassembly 34. The hook assembly 34 engages the wire 14 a to support theconductors 16 a. The structure of the wire 14 a is identical to thatshown in FIGS. 2 and 3 with external wire strands wrapped extendingalong a plastic tube that holds a bundle of fibers.

[0028] The plastic tube 18 resists the radial loads imposed by the hookassembly 34 with the strands providing the requisite tensile strength.

We claim:
 1. A neutral or messenger wire for use in an above groundpower distribution system, said neutral wire comprising an elongateplastics tube, at least one optical fiber freely moveable within saidtube and having an overall length not less than 0.05% greater than saidtube in a free body state, and a plurality of metal wire strandsdisposed about said tube and extending along the length thereof.
 2. Aneutral wire according to claim 1 wherein a cladding is applied to saidtube and is of a different plastics material.
 3. A neutral wireaccording to claim 2 wherein said cladding is polyethylene.
 4. A neutralwire according to claim 1 wherein said strands are aluminum alloy.
 5. Aneutral wire according to claim 1 wherein said strands are galvanized.6. A neutral wire according to claim 1 wherein said strands have analuminum cladding.
 7. A neutral wire according to claim 5 wherein saidtube is clad with a plastics material.
 8. A neutral wire according toclaim 1 wherein said plastic tube exhibits a longitudinal elasticitygreater than 1%.
 9. A neutral wire according to claim 1 wherein saidplastic tube is formed from a plastic selected from the group consistingof polycarbonate, polybutylene terephtalate, polyethylene terphthalateand polyethylene napthalate.
 10. A neutral wire according to claim 1wherein said plastic tube has a wall thickness sufficient to withstandradial loads imposed thereon during installation.
 11. A neutral wireaccording to claim 10 wherein said plastic tube has an outside diameterbetween 3 mm and 7 mm and a wall thickness of between 0.5 mm and 2 mm.12. A neutral wire according to claim 11 wherein said plastic tube has acladding of polyethylene.
 13. A ground wire according to claim 12wherein said polyethylene has a thickness of 0.1 mm to 1.0 mm.